Ultra high frequency oscillator



Mlm m W43- W. VAN B. ROBERTS Em ULTRA HIGH FREQUENCY OSCILLATOH FiledAug. 22, 1940 nte-med Jan. l2, 1943 ECE ULTRA man FREQUENCY oscILLAToRWalter van B. Roberts, Princeton, N. J., assigner to Radio Corporationof America, a corporation of Delaware Application August 22, 1940,Serial No. 353,626

(Cl. Z50- 36) Claims.

The present invention relates to improvements in ultra high frequencyoscillators.

It is known in the art to velocity modulate or otherwise effectivelybreak up into groups an electron beam and to cause the beam (so grouped)to traverse a portion of space associated with a tank circuit, more orless along at least some of the lines of force thereof, in which analternating electric field exists, and to collect the emerging beam on aseparate electrode. If the grouping of the electrons corresponds to thenatural frequency of the tank circuit, alternating power is absorbedfrom the electron beam by the tank, while the actual electrons all go tothe collector. This collector may be relatively large so as to dissipatethe heat created by electron impact. Such a known arrangement isreferred to in the art as an inductive output tube and is described inan article by A. V. Haeff, published in Electronics, February, "1939,and in an article by A. V. Haeii and L. S. Nergaard, published inthefProc. of the I. R. E, March, 1940.

It is also known in such a system to maintain oscillations by utilizingthe electrostatic field resident in another portion of the tank or in aseparate circuit coupled thereto to cause the aforesaid electrongrouping, as by feed-back effects. In the simplest form of tankcircuits, however, namely, in the concentric line having a length equalto one-quarter of the length of the operating wave and short-circuitedat one end, this last known scheme cannot be used without the additionto the quarter wave tank of further circuit elements. This last form oftank circuit is characterized by a single fundamental degree of freedom.

In accordance with the present invention, the dimensions of the tankcircuit are minimized and the construction thereof simplified byutilizing the magnetic field associated with a part of the tankstructure for controlling the direction of the electron beam andabstracting power from the beam not in virtue of grouping of theelectrons but as a result of transverse deflections thereof.

The preferred embodiment of the invention is an ultra high frequencyoscillator comprising a tank circuit having associated therewith a spacein which there is a relatively strong magnetic field due to oscillationsin the tank and a relatively weak electrostatic field, and another spacecharacterized by a relatively weak magnetic field but a relativelystrong electrostatic field, and an electron beam or stream arranged totraverse said spaces in the order named, and furthermore to traverseboth of said fields substantially normally to at least a portion of thelines of force of each, and finally a collector electrode separate fromsaid tank for collecting said electron beam after its passage throughthe said fields.

The detailed operation of the invention, however, will best beunderstood by reference to the accompanying drawing, wherein:

Fig. 1 shows the invention in its simplest form;

Fig. 2 shows a modification of the system of Fig. 1, wherein the tankitself is not required to be within the evacuated envelope;

Fig. 3 shows a further modcation of the form of tank circuit and also anassociated horn type of wave radiator;

Fig. 4 is a further modification of the invention employing a concentricline type of resonator or tank; and

Fig. 5 shows another modification which the tank circuit may take.

Referring now to Fig. 1, there is shown within an evacuated envelope I0a quarter-wave (L) Ll tank I composed of two strip or ribbon shapedvconductors l', I connected together at one end by a metallic member 2having an aperture therethrough for the passage of the electron beam 3emanating from gun 4. Gun 4 may be a cathode or any suitable element forproducing and projecting a stream of electrons. Member 2 is of goodelectrically conducting material and is located at a voltage nodal pointof the tank. An electrode 5 serves to collect the electron beam after ithas traversed the length of the tank. The theory of operation will nowbe given: Let us assume oscillations to be present starting frominevitable infinitesimal random causes and building up to a limitingamplitude (as is the case where the feed back mechanism is effective atsmall amplitudes); and let us assume, furthermore, that the electronbeam velocity has been adjusted so that the time of fiight through thetank is a half period of the oscillation. The time of flight may be madea multiple of this value, but for simplicity of explanation a halfperiod of time of flight of the electrons will be assumed. Let us nowconsider an electron which passes through the aperture in member 2 attime t, the instant when the tank current is maximum. Since at thisinstant the magnetic field between the two conductors I', I', is amaximum and is perpendicular to the plane including the longitudinalcenter line of the two strip conductors I', I (i. e., plane of thepaper), the electron will be deected sideways toward one or the other ofthe conductors I', l. Assume that the deflection is to the right. A halfperiod later the electron will therefore pass the open end of the tank,and the potential of the open ends of the tank will be at zero. Supposefor the moment that the potential of the right end had been negativejust prior to the emergence of the electron from the tank and that thispotential changes to positive immediately thereafter. A suitableadjustment of the time of flight will always be possible that will makethe potential of the end of the tank nearest to which the electronpasses change in the sense stated above as the electron passes. In anycase, the phase of this change can always be reversed by giving the pairof conductors a half twist between the two ends of the tank; or, whatamounts to the same thing, by criss-cross connecting the two open endsof the tank to the two shorted ends thereof in order to obtain thisrelation. Thus, it will be seen that the electron in departing from thecenter line as it approaches the end of the tank, has been travellingagainst the crosswise force acting upon it and has been slowed down.After the electron emerges from the tank and approaches the collectorelectrode 5, it also moves against the potential gradient, since thepotential of the end of the tank reverses just as the electron passes.Thus, energy is absorbed from the electron into the tank and tends toreinforce the oscillations assumed therein. Similarly, it may be shownthat an electron passing the aperture in plate member 2 half a periodlater will also give up energy to the tank. In the ideal case, thegreatest eiiiciency would be obtained by permitting the beam to pass theaperture only during short intervals in the vicinity of the successivetimes when the tank current is maximum, in a manner analogous to theflow of space current of a class C vacuum tube amplifier of the usualtype. Even without recourse to any such expedient, the integratedexchange of energy over a whole cycle between the beam 3 and the tank Igives a net increase of tank energy at the expense of electron energy,thus main,

taining oscillations if the tank losses per cycle are kept less than theenergy fed back per cycle.

In the tank of Fig. l, there is a relatively strong magnetic eld and aweak electrostatic field in the vicinity of the short circuiting member2, and a weak magnetic field and a strong electrostatic eld in thevicinity of the open ends of the tank circuit. Thus, it will be apparentthat the electrons in their travel pass rst through the strong magneticfield with its corresponding weak electrostatic field and then throughthe weak magnetic eld and the strong electrostatic eld.

Fig. 2 shows how the electron beam and the collector electrode may becontained in an evacuated glass tube I [l which extends between the tankmembers. This construction of having the tank members l', I outside theenvelope avoids the necessity of degassing the tank elements.

Fig. 3 shows a modified tank structure and also an associated hornstructure for promoting radiation of waves in a desired direction. As tothc tank, the side members l', I' are folded at l so as to decrease thedistance the beam charges must travel in going from the preponderantlymagnetic eld portion ot' the tank to the preponderantly electrostaticportion while keeping the natural frequency of the tank unaltered. Thispermits operating the electron beam ai lower velocities. The open endsof the tank sides i', l may also be bent apart, as shown, to promoteradiation from the tank or may be bent together to reduce radiation, ifdesired, in the manner shown in Fig. 5. Fig. 3 is also provided with ametal rectangular or cylindrical box 6 enclosing the tank I', l. Thisbox has expanding dimensions and forms a horn type radiator for thepower fiowing out as radiation from the end of the tank. The collectorelectrode 5 is here arranged to have such a shape as not to interferewith the radiation from the tank,

Fig. 4 shows a two conductor type of tank circuit embodying the sameprinciples of the invention. In this figure, the tank is a quarter waveconcentric affair of a single degree of freedom having in its shortcircuiting member 2' a substantially annular slot 8 which is broken by afew radial connections between the inner and outer conductors. Theelectrons pass through the slot 8 and may emanate from the gun as ahollow tubular electron beam.

In Fig. 5 the tank circuit strips are connected together at one end byan apertured member which permits the electrons to enter therein, andhave at their other ends a pair of plates 9, 9 to permit the electronsto emerge.

What is claimed is:

1. The method of sustaining oscillations in a single tank circuit havingstatic and magnetic elds produced by said oscillations, the magnetic eldbeing preponderant in one portion only of the tank and the electric eldbeing preponderant in another portion only of the tank, which comprisespassing a beam of charges through said fields in succession in the ordernamed in a direction such that the chief ccmponent of each of saidfields is transverse to the said beam, and collecting the beam emergingfrom said elds.

2. In an oscillator, a single resonant tank circuit therefor which ischaracterized by static and magnetic fields preponderantly associatedrespectively with diferent portions of the tank circuit, said magneticfield portion comprising a closed conductor having an aperture in thecenter thereof, said tank circuit being formed of two substantiallyparallel conductors, means for causing electrons in the form of spacecurrent to pass through said aperture and between said conductors overthe entire length of said tank circuit to thereby traverse said fieldsin succession, said tank circuit having such dimensions that the motionof said electrons is controlled by the first of said fields to betraversed and the reaction between said controlled electrons and saidother field transfers oscillatory energy from said electrons to saidtank circuit.

3. A single resonant electrical tank having a closed end and a singleopen end and characterized by a transverse magnetic eld preponderantlyassociated with the closed end and an electric eld preponderantlyassociated with the open end, means for projecting a beam of chargesthrough said magnetic field in the region where the lines of forcethereof are transverse to the direction of the beam and then forprojecting said beam transversely through said electric eld, and meansfor adjusting the velocity of said beam to cause power to be absorbed byway of said electric iield by said tank.

an oscillator, an oscillatory circuit comprismg a pair of conductorselectrically connected together at one end and open at the other end,said conductors each being eectively one-quarter of a wavelength long atthe operating frequency, means for projecting a beam of charges throughsaid oscillatory circuit from said closed end to said open end, andmeans for collecting the charges emerging from the open end of saidoscillatory circuit, the time of flight oi the charges through saidoscillatory circuit being substantially an integral multiple includingunity of a half period of the oscillation, whereby the magnetic eldproduced by the oscillatory energy in said oscillatory circuit deectssaid beam and energy is absorbed from the charges in said beam as theyemerge from the open end of said oscillatory circuit for reinforcing theoscillations in said oscillatory circuit.

5. An oscillator in accordance with claim 4, characterized in this thateach of said conductors is folded in order to decrease the distance saidbeam travels through said tank.

6. An oscillator in accordance with claim 4, characterized in this thatthe oscillatory circuit is in the form of a coaxial resonator and thatthe beam of charges takes the form of a hollow tubular electron stream.

7. A resonant electrical tank having a magnetic eld preponderantlyassociated with one portion and an electrical field preponderantlyassociated with another portion, said one portion comprising a closedend having an aperture therein, means for projecting a beam of chargesthrough said aperture and then transversely through said electric iield,whereby said charges travel transversely through both of said fields insuccession, and means for adjusting the velocity of said beam to causepower to be absorbed by said tank from said beam.

8. In an oscillator, an oscillatory circuit comprising a pair ofribbon-like conductors extending in the same general direction for thegreater part of their lengths and electrically connected together at oneend and open at the other end, said conductors each being effectivelyone-quarter of a wavelength long at the operating frequency, means forprojecting a beam of charges between said conductors from said closedend to said open end, and a collector electrode for collecting thecharges emerging from the open end of said oscillatory circuit, the timeof flight of the charges through said oscillatory circuit beingsubstantially an integral multiple including unity of a half period ofthe oscillation, whereby the magnetic field produced by the oscillatoryenergy in said oscillatory circuit delects said beam and energy isabsorbed from the charges in said beam as they emerge from the open endof said oscillatory circuit for reinforcing the oscillations in saidoscillatory circuit.

9. An oscillator in accordance with claim 8, characterized in this thatan evacuated envelope surrounds said oscillatory circuit, said beamprojecting means. and also said collector electrode.

1Q. An oscillator in accordance with claim 8, characterized in this thatan evacuated envelope surrounds said beam projecting means and saidcollector electrode, and is located between the conductors of saidoscillatory circuit.

11. An oscillator comprising a single oscillatory circuit in the form cia pair of substantially parallel conductors connected together at oneend bv a metallic plate and open at the other end. said plate having anaperture therein. an electron gun outside of said oscillatory circuit atone end thereof for proiecting a beam of electrons through saidaperture, a collector electrode also outside said oscillatory circuitand at the other end there of for collecting the electrons traversingthe length of said circuit, said conductors having such length thatthere exists a strong magnetic field and a weak electrostatic field inthe vicinitylof said plate and a weak magnetic eld and a strongelectrostatic eld in the vicinity of the open end of said oscillatorycircuit, the chief component of each of said strong fields beingtransverse to the direction of travel of said beam, and the velocity ofthe electrons in said beam being such that said beam in its travelpasses through both of said strong elds in succession.

12. A resonant electrical tank having a single fundamental degree offreedom and characterized by a eld preponderantly magnetic associatedwith one portion and a iluid preponderantly electric associated withanother portion, said magnetic field portion comprising a closedconductor having an aperture therein, means for projecting a beam ofcharges through said aperture and then transversely through saidelectric eld, whereby said charges travel transversely through both ofsaid iields in succession, and means for adjusting the velocity of saidbeam to cause power to be absorbed by said tank from said beam.

13. A resonant electrical tank having distributed inductance andcapacity and inherently having associated with one portion a eld whichis preponderantly magnetic, and with another portion a eld which ispreponderantly electric, said magnetic field portion comprising a closedconductor having an aperture therein, and means for projecting a beam ofcharges through said aperture and then transversely through saidelectric field, whereby said charges travel transversely through both ofsaid elds in succession, the length of said tank being so related to theVelocity of the charges that power is absorbed from the beam at thenatural frequency of the tank.

14. A resonant electrical tank having distributed inductance andcapacity and inherently having a magnetic eld preponderantly associatedwith one portion, and an electrical eld preponderantly associated withanother portion, means for projecting a beam of charges transverselythrough said elds in the order named, the length of said tank being sorelated to the Velocity of the charges that power is absorbed from thebeam at the natural frequency of the tank, and a horn type radiatorsurrounding said tank and having dimensions expanding from thepreponderantly magnetic eld portion to the preponderantly electric eldportion.

15. In an oscillator, an oscillatory circuit comprising a pair ofconductors electrically connected together at one end and open at theother end, said conductors each being effectively one-Quarter of awavelength long at the operating frenuency, means for projecting a beamof charges through said oscillatory circuit from said closed end to saidopen end, and means for collecting the charges emerging from the openend of said oscillatory circuit, the time of flight of the chargesthrough said oscillatory circuit being substantially an integralmultiple including unity of a half period of the oscillation, wherebythe magnetic iield produced by the oscillatory energy in saidoscillatory circuit deects said beam and energy is absorbed from thecharges in said beam as they emerge from the open end of saidoscillatory circuit ror reinforcing the oscillations in said oscillatorycircuit, and a horn type radiator surrounding the conductors of saidoscillatory circuit and having dimensions expanding from the connectedend of the conductors to the open end thereof.

WALTER VAN B. ROBERTS.

